If the current understanding of cancer is correct, there must be something fundamentally different in large, long-lived organisms that enhances their suppression of carcinogenesis. These mechanisms have allowed for the evolution of large bodies and extended lifespans without increasing the burden of cancer. Most of the hypotheses that have been proposed have not been directly tested, and most related questions remain open (Box 3
Box 3. Outstanding Questions for Peto’s Paradox
- What is the age-related incidence of cancer in most non-human (and non-experimental) animal species?
- Which of the many suggested mechanisms are valid explanations for the lack of correlation between body size, longevity and cancer incidence? Compared to humans, do larger, long-lived organisms have:
- Lower somatic mutation rates?
- More copies of tumor suppressor genes?
- Fewer proto-oncogenes?
- Smaller selective advantages for somatic mutants?
- Different tissue architecture (smaller proportion of stem cells or more quiescent stem cells)?
- More efficient immune surveillance?
- An apoptotic process highly sensitive to DNA damage?
- Increased sensitivity to contact inhibition?
- Shorter telomeres?
- Less DNA damage due to fewer reactive oxygen species?
- Is the presumed decrease in cancer incidence in lineages with lower than expected cancer incidence the result of several mechanisms that each contribute in a cumulative manner to decreasing the cancer risk of each cell, or rather of a single mechanism that has a drastic effect on a cell’s probability of becoming malignant?
- Are such mechanisms shared among large, long-lived species or are they unique to each species?
- Does the cancer protection come from some innate characteristic of large organisms (i.e. low mass specific basal metabolic rate)?
- Can the cancer suppression mechanisms used by large, long-lived organisms be translated to humans as novel cancer preventive interventions?
Large bodies have evolved independently multiple times in the history of life, so each clade could have evolved different mechanism(s) to boost their tumor suppression abilities. An approach based on independent contrasts [60
] of small and large species within each clade could prove fruitful for identifying cancer suppression mechanisms (Box 4
Box 4. A Phylogenetic Approach to study Peto’s Paradox
Large, long-lived organisms might have evolved to suppress cancer better than small animals by duplicating tumor suppressor genes [2
] or eliminating some proto-oncogenes from the genome. A simple linear regression cannot be used to study whether a correlation exists between body size and the copy number of cancer related genes because this assumes independence of each genome. In reality, the genomes share many traits in common due to evolutionary decent from a common ancestor. An independent contrast model [73
] should be used to partition the variance among species into comparisons that are independent of their evolutionary relationships. This can be done by studying multiple clades, each composed of closely related species which have large variance in body size. Marine mammals belonging to the Order Cetacea
are an ideal clade for this study since they range in size from small dolphins like the Commerson’s dolphin (~50 kg) [35
] to the largest mammal on Earth, the blue whale (over 100,000 kg) [35
]. The split between dolphins and whales occurred only 25–30 million years ago [74
]. Unfortunately, the genomes of these animals are not currently available. Studies should focus on clades that include animals larger than humans, as opposed to looking at differences among various sized rodents or between mouse and human, because the goal is to find a way of preventing cancer that is superior to endogenous tumor suppression mechanisms in humans.
Unfortunately we currently lack data necessary to make these analyses possible. Efforts should focus on sequencing genomes of large, long-lived species along with closely related small species, to determine if tumor suppressor genes duplicated, or oncogenes were deleted, during the evolution of a large body within a clade. Gene expression analyses of the same tissues might also reveal differential expression of cancer genes in large organisms. In addition there are standard assays that could be used in comparative analyses to test many of the hypotheses for resolving Peto’s paradox (Box 5
), including measurements of DNA damage repair [61
], telomere lengths [62
], differentiation [63
] and proliferation [64
], apoptosis [66
], and reactive oxygen species [67
Box 5. Suggestions for Future Experiments
There are many experiments that could be done to test the hypotheses proposed to explain Peto’s Paradox; however, they are limited by currently available information and assays as well as the fact that large, long-lived animals cannot be easily genetically manipulated in a laboratory.
- Lower somatic mutation rates: Mutation rates can be measured in elephant and whale cells in vitro; however with better assays and longitudinal tissue sampling, the in vivo somatic mutation rate could be estimated .
- More copies of tumor suppressor genes or fewer proto-oncogenes: Copy number of cancer-associated genes can be studied using genomics to count the orthologs of known cancer genes using independent contrasts (Box 4). This is based on sequence information only, so functional studies would be necessary as follow-up.
- Smaller selective advantages for somatic mutants: Fitness affects of mutations in cells of different animals might be estimated using in vitro cell competitions; however this is not a realistic environment and might not reflect the true fitness caused by the mutation in vivo. Modern genetically engineered organisms could be used to measure the fitness of isolated mutations in vivo [76, 77].
- Different tissue architecture (smaller proportion of stem cells or more quiescent stem cells): The mitotic index could be measured for crypts in intestinal tissue samples from elephant and whale. Given reliable stem cell markers, stem cells could also be counted.
- More efficient immune surveillance: It might be possible to measure the immune response to mutant proteins that vary from the endogenous sequence by different degrees.
- An apoptotic process highly sensitive to DNA damage: Cells from animals like elephants and whales can be irradiated in vitro to quantify how many cells apoptose as a function of the amount of DNA damage.
- Increased sensitivity to contact inhibition: Cells can be grown in vitro to determine how the density of the cultures when the cells stop growing compares to the density of cultured cells from smaller organisms, as was done with the naked mole-rat .
- Shorter telomeres: Telomere lengths can be measured and compared across species by standard assays.
- Less DNA damage due to fewer reactive oxygen species: New methods involving fluorgenic sensors for superoxide and hydroxyl radicals can detect ROS in cell cultures, tissues and in vivo .
The majority of cancer research is done on a very small subset of organisms which restricts our understanding of cancer to what we learn from those particular model systems. Furthermore, the qualities of model organisms that make them ideal to work with in laboratory conditions (short lifespan and small body) are the very things that make them poor models for cancer suppression [2
]. The lack of functional data for non-model organisms is a major gap in the field. Function is often assumed from homology which is not necessarily correct. For example, TSGs in Drosophila
are largely non-overlapping with human tumor suppressors [68
]. Studies that aim at a better understanding of the evolution of cancer suppression mechanisms will have to expand the variety of organisms that are studied in the laboratory setting.
There are not many large, long-lived organisms that have been fully sequenced yet, so testing Peto’s Paradox by doing comparative genomic analyses is difficult with current data. We are also lacking robust epidemiological studies of cancer incidence in wildlife and captive populations. Captive populations will be useful for longitudinal studies and the predation-free environment will allow for better estimates of cancer rates. This will help researchers to better understand the nature of Peto’s Paradox.